The objective of this research is to study the nature and functions of antibodies responsible for the sensitization-inhibiting effects of immunological enchancement-type tolerance, and to elucidate the mechanisms of the induction of delayed-type hypersensitivity especially as affected by these antibodies. This objective will be accomplished by using well-established and characterized models of tolerogenesis and sensitization in mice, with purified antigens and defined antigenic determinants, and recently developed partially in vitro adaptions of these models. Specific goals relating to the nature of enhancement-type tolerance include identifying the immunoglobulin class(es) of sensitization-suppressing antibodies, characterizing the antibodies immunochemically (determinant specificity, affinity, integrity), and delineating their immunobiological attributes (source of production, site of reaction, effect of interaction with antigen and immunocytes resulting in immunosuppression, conditions favoring their production and action). Additional goals relating to induction of delayed hypersensitivity include dissecting the events of induction of vitro (types of lymphoid cells involved and their functions, mode of antigen presentation, effects of immunosuppressive antibody and of cells making this antibody) and then translating in vitro findings to in vivo significance by using these findings to design and perform analogous experiments in mice.